The tendency of a metallic structure to corrode in a marine environment is known. This corrosion is caused primarily by an electrochemical or chemical reaction between the metal and the water. The corrosion is particularly acute when the system is operated in salt water.
A known system for prevention of corrosion of a marine system includes a sacrificial anode which is electrically connected to the propulsion system and is in contact with the water. This anode is typically made of zinc, and occupies a position in the Galvanic series with respect to the material of the propulsion system to be protected (e.g., an aluminum alloy) such that the parts of the propulsion system to be protected become cathodes. The material of the sacrificial anode dissolves into the water and is then deposited on the material of the propulsion system.
An example of such a system utilizing such a sacrificial anode is disclosed in unexamined Japanese application 63-100187.
A significant problem with systems using a sacrificial anode is that the material of the anode is deposited on the surface of the protected system. In the case of a marine propulsion system with a propeller having a shiny surface, for example one made of stainless steel, these deposits are unsightly and detract from its efficiency.
In addition, when the propulsion system to be protected is large, the amount of material to be protected becomes large, and the sacrificial anode must be accordingly large. Moreover, the protection of the material of the system becomes more difficult as the distance from the sacrificial anode increases.